1. Field of the Invention
The present invention relates to a closed loop type air-fuel ratio control system of an internal combustion engine, which controls the air-fuel ratio of the engine inlet mixture to a desired value in accordance with the concentration of a selected component of the exhaust gas from the engine.
2. Description of the Prior Art
In order to reduce the harmful compounds, such as HC, CO and NO.sub.x, contained in the exhaust gas from the internal combustion engine, a so-called three-way catalytic converter is widely used. In this connection, it is known that the three-way catalytic converter exhibits its maximum function against those three harmful compounds when the air-fuel mixture to be fed into the engine has a stoichiometric air-fuel ratio. In case of gasoline powered internal combustion engine, the stoichiometric air-fuel ratio is about 14.7:1.
As a system which puts the above-mentioned fact into a practical use, a closed loop type air-fuel ratio control system has been employed in the engine system, which generally comprises an electrically controlled carburetor arranged in an intake line of the engine, an exhaust gas sensor (such as an oxygen sensor) arranged in the exhaust line of the engine at a position upstream of the converter for issuing an information signal representative of the air-fuel ratio of the inlet mixture actually fed into the engine proper, and a control unit electrically connecting to both the carburetor and the sensor for controlling the operation of the carburetor in accordance with the information signal issued from the sensor. However, some of the control systems of this type fail to exhibit the precise function throughout the whole operation modes of the engine, as will become clear hereinafter.